The present invention refers to an antenna system for a mobile terminal, according to the preamble of claim 1.
In mobile terminal technology, there has been, and continues to be, a trend towards decreasing the size of the terminals themselves. There is also a requirement for the antenna system to be equipped to use two or more widely separated frequency bands.
Combined antennas, with multiple resonances at the desired frequencies, are known to persons skilled in the art. External antennas are usually designed as removable mechanical units, resonant in two frequency bands. Built in antennas for the cellular phone bands are often of PIFA (Planar Inverted F Antenna) or patch type. External antennas and built-in antennas of the types mentioned use matching of 50 ohms included in the antenna element structure. A disadvantage with these antennas, is that they are not optimal in size for employing two or more resonances at frequencies used for communication between mobile terminals and base stations.
U.S. Pat. No. 4,491,843 describes an antenna for a portable receiver, comprising a metal plate and a metal box or case, whereby the antenna forms an electric dipole, comprising the metal plate and another xe2x80x9cvirtualxe2x80x9d metal plate, located half-way up the box. A disadvantage with the design in U.S. Pat. No. 4,491,843 is that the electric dipole formed is small compared to the wavelength. The effect of this is that the size of the band-width that is possible to obtain is very limited. Another disadvantage is that in order for the design in U.S. Pat. No. 4,491,843 to function as a electric dipole, a number of geometric requirements has to be fulfilled, e.g. the size and shape of the metal box and metal plate, and the distance between them. This limits the possibilities for variations in the arrangement and design of the mobile terminal.
It is an object of the present invention to provide an antenna system for a mobile terminal, which facilitates the design of small terminals and is more effective and more cost effective than present antenna systems.
The object is met by an antenna system, having the characterizing features of claim 1.
Using an end-fed antenna located within the casing of the terminal, and having a counterpoise element, located near one end of the end-fed antenna, results in less restrictions in the designing of the mobile terminal, since the counterpoise element, in order to meet antenna requirements, can assume a wide range of shapes and sizes. This means that the invention opens new possibilities for the creation of different industrial designs.
Preferably, the end-fed antenna extends through a major part of the mobile terminal casing, and is formed by the terminal chassis or-the screening device for the radio electronic circuits. Alternatively, the casing itself forms an antenna. Whether the chassis, screening or casing is used, the effect is that essentially the full mechanical length of the terminal, i.e. as much as possible of the available space is used for the antenna. This has the result of presenting a larger antenna than in known terminals, which provides for a larger bandwidth. The fact that the bandwidth of a small antenna is limited by its size is explained by Balanis, xe2x80x9cAntenna theory analysis and designxe2x80x9d, pages 566-571, John Wiley and Sons.
Preferably, the electrical length of the end-fed antenna is in the vicinity of a full or a half wavelength of a frequency, on which the end-fed antenna is intended to transmit or receive. The electrical length of an antenna is often slightly larger than the physical length of the latter, as explained, for example in xe2x80x9cThe Radio Amateur""s Handbook 1967xe2x80x9d by ARRL, page 369. Providing a full or a half wavelength antenna makes it possible to use a counterpoise with a lower self-capacitance, than what would have been the case for a quarter wavelength antenna. This means that the counterpoise can be small in size.
Further features, developments and advantages with the present invention are obtained in connection to the dependent claims, and are revealed in the description here below.